In recent years, there has been desired a smaller and more power-saving actuator than an electromagnetic actuator, and a piezoelectric actuator including a piezoelectric element as a vibration source has been being developed. The piezoelectric actuator generates mechanical vibrations by movement of the piezoelectric element. The piezoelectric actuator can be utilized as an audio component, such as a vibrator used for a mobile device, and a loudspeaker.
However, there is a problem that rigidity of the piezoelectric element is generally high, and that amplitude of vibrations of the piezoelectric actuator is smaller than amplitude of vibrations of the electromagnetic actuator. In order to solve this problem, a technology for increasing the amplitude of the vibrations of the piezoelectric actuator has been disclosed in the following Patent Literatures 1 to 5.
Patent Literature 2 discloses a piezoelectric actuator including a vibrating body (pedestal) formed by connecting an elastic body and a piezoelectric element. In this piezoelectric actuator, a periphery of the elastic body is supported by a spring structure having a resonance frequency close to a resonance frequency of the piezoelectric element. In a piezoelectric actuator described in Patent Literature 3, a slit is formed in an elastic body (pedestal) to which a piezoelectric element is connected, and as a result, amplitude of vibrations of the elastic body is amplified.
In Patent Literature 4, an audio device including a piezoelectric actuator has been disclosed. In this piezoelectric actuator, the periphery of a piezoelectric vibrator formed by connecting a piezoelectric element and a vibrating diaphragm (pedestal) is fixed to an inner periphery of a ring-shaped support member. Additionally, an outer periphery of the support member is fixed to a case. A portion between the inner periphery and the outer periphery of the support member is curved. The piezoelectric actuator can vibrate in a direction vertical to a plate surface of the vibrating diaphragm, and in a direction parallel to the plate surface of the vibrating diaphragm by connecting the pedestal to the case through the curved support member.
In a piezoelectric actuator described in Patent Literature 5, a piezoelectric element is connected to both surfaces of one restraining member (pedestal). The piezoelectric element is restrained by the restraining member. A plurality of beam members are formed on an outer periphery of the restraining member, and these beam members are fixed to a support member.
Incidentally, a piezoelectric vibrator of a piezoelectric actuator is classified into a bimorph type vibrator in which a piezoelectric element is connected to both surfaces of one pedestal, and a unimorph type vibrator in which the piezoelectric element is connected only to the one surface of the one pedestal. In Patent Literatures 1, 2, 4, and 5, the bimorph type piezoelectric vibrator has been disclosed. Generally, the bimorph type piezoelectric vibrator has a higher drive force and a larger amplitude of vibrations than the unimorph type piezoelectric vibrator.
FIG. 1 is a schematic exploded perspective view of a typical bimorph type piezoelectric actuator. Piezoelectric actuator 3200 has piezoelectric element 3212 comprised of piezoelectric ceramics, pedestal 3216 to which piezoelectric element 3212 is connected, and frame-shaped support body 3210 that supports pedestal 3216. Pedestal 3216 is plate-shaped, and an outer periphery of the pedestal is supported by support body 3210.
FIG. 2 is a schematic view schematically showing a condition of vibrations of piezoelectric actuator 3200. When an AC (alternating-current) voltage is applied to piezoelectric element 3212, it performs expansion and contraction movement. Since piezoelectric element 3212 is connected to pedestal 3216, pedestal 3216 bends in direction R vertical to a principal surface thereof according to the expansion and contraction movement of piezoelectric element 3212 as shown in FIG. 2. In a manner described above, pedestal 3216 vibrates in direction R vertical to the plate surface of the pedestal in a state where a connection portion with support body 3210 is set as a fixed end and a center of pedestal 3216 is set as a belly.
It is assumed that the piezoelectric actuators described in Patent Literatures 1 to 3 are mainly utilized as vibrators. Namely, in the piezoelectric actuators described in Patent Literatures 1 to 3, there is no intent to improve characteristics as audio components.
In the case where the piezoelectric actuator is used as the vibrator, the amplitude of vibrations of the piezoelectric actuator may be large only in a specific frequency. However, in a case of using the piezoelectric actuator as an audio component, frequency characteristics of the piezoelectric actuator are important. It is desirable for the audio component not only to have a high sound pressure level, but also to have flat frequency dependence of the sound pressure level in order to faithfully reproduce the original sound. It is desirable that the frequency dependence of the sound pressure level is flattened in an audible region, particularly in a frequency band of 100 Hz to 10 kHz.
In an audio component including a piezoelectric vibrator, the bimorph type piezoelectric vibrator is widely used in order to secure a drive force of the piezoelectric vibrator and to improve the sound pressure level. However, the bimorph type piezoelectric vibrator has higher rigidity than the unimorph type piezoelectric vibrator, and as a result of it, a frequency of amplitude of vibrations of the piezoelectric actuator is extremely changed near a resonance frequency.
In the piezoelectric actuator described in Patent Literature 4, since the piezoelectric vibrator is not strongly fixed to the case, the frequency change of the amplitude of the vibrations of the piezoelectric actuator is flattened. However, since energy is used for vibrations in a radial direction of the vibrating diaphragm in the vibrating diaphragm of the piezoelectric actuator, the amplitude of vibrations in a thickness direction of the vibrating diaphragm becomes low. Hence, there is a problem that the sound pressure level is lowered when this piezoelectric actuator is utilized as the audio component.
The invention in Patent Literature 5 aims at providing a piezoelectric actuator in which amplitude of vibrations is large and in which the resonance frequency can be adjusted while avoiding increase of an outside dimension. However, it is desirable to improve the sound pressure level in a low frequency band and to flatten frequency characteristics of vibration amplitude for the purpose of higher performance.
In addition, when the piezoelectric actuator is utilized as the audio component, further improvement of the piezoelectric actuator is desired in order to achieve improvement of the sound pressure level and flattening of the frequency characteristics of the sound pressure level as described above.
What is desired, in addition, is a piezoelectric actuator in which the vibration characteristics are easy to adjust and in which there is a high degree of design freedom so that a piezoelectric actual can be provided that has various applications, such as being used as a vibrator, rather than having only an application as an audio component.